|
The onset of flowering in plants represents a major
developmental shift in the plant life cycle. This shift
is controlled by environmental cues, physiological changes,
and alterations in gene expression. Many of the genes
involved in the induction of flowering have been identified
in model organisms. At the shoot apex, a small number
of floral meristem identity genes control the positioning
and identity of flowers. In Arabidopsis, flowers are
organized into four whorls. The organs on each of these
whorls are specified by combinations of transcription
factors, which are encoded by MADS box genes. These
genes originally were identified by studying homeotic
mutant phenotypes in which whorl identity is altered.
The MADS box genes that control the patterning of floral
organ identity evidently are conserved among angiosperms.
Gametes must be formed
to complete the plant life cycle. In the anthers, sporogenous
cells undergo meiosis and each resulting haploid cell
eventually develops into a male gametophyte (pollen
grain). In the ovules, sporogenous cells undergo meiosis
and eventually form the female gametophyte (embryo sac).
Many genes are expressed only during gametophyte development.
When pollen grains contact the stigmatic tissues of
the gynoecium, they hydrate and then extrude a pollen
tube, growing though stylar tissue to deliver the two
sperm cells to the embryo sac. Typically, one sperm
cell nucleus fuses with the egg cell nucleus and the
other fuses with the nuclei of the central cell. Genetic
analysis of mutants and molecular analysis of gene expression
have contributed to our understanding of gametophyte
development and pollen–pistil interactions.
Seed development proceeds
from the single-celled fertilized egg to a multicellular
structure comprising an embryo, which is the next generation
of the plant; the seed coat (testa); and a nutritive
endosperm that persists in the mature dry seed of some
species. Many genes are involved in the regulation of
seed development. The roles of some of these genes have
been elucidated by using developmental mutants. An important
aspect of seed development is the deposition of the
stored reserves—carbohydrates, proteins, and oils—that
are utilized after germination to support early seedling
growth. The terminal phase of seed development is desiccation,
which results in a mature dry seed. On subsequent imbibition
of water, metabolism is renewed as the seed commences
germination, which is completed with the emergence of
the radicle.
|
